The field of the invention is that of detectors with quantum well structure working in the infrared range, both in band II (between 3 and 5 .mu.m) and in band III (between 8 and 12 .mu.m).
At present, there are three types of detectors used in the infrared domain.
A first type relates to the detectors with Schottky junction formed between silicon and platinum silicon (Si/PtSi). These detectors perform well (as measured by a minimum temperature detectable commonly called NETD or noise equivalent temperature detectable of about 80 mK in band II for reading frequencies of 50 Hz, surface areas of 50 .mu.m.times.50 .mu.m and an optical aperture of f/2), but have very low working temperatures (of about 77 K).
A second type relates to bolometrical or pyrometrical thermal detectors based on the changing of electrical properties such as the dielectric index or the resistance under the effect of heating due to the radiation absorbed. These detectors lead to more modest performances (with NETD values of about 120 mK under the conditions of measurement described here above and for band III). Nevertheless, their great value is that they work at ambient temperature without needing to be cooled.
A third type of detector relates to quantum structure detectors using p-n junctions based on semiconductors such as InSb, HgCdTe or PbSnSe or intersubband transitions in quantum wells of materials such as GaAs/AlGaAs.
These detectors have very high performance characteristics (with NETD values in the neighborhood of 20 mK) but work at temperatures in the range of 200 K in the best cases.
These last-mentioned detectors which perform very well therefore require cyrogenic cooling. This represents an extra cost for the equipment.
Indeed, as described in R. J. Keyes, "Optical and Infrared Detectors", Springer-Verlag, the detectability of a component of this type is proportional to I/.sqroot.I.sub.dark where I.sub.dark is the dark current of the detector. In semiconductor-based quantum structure detectors, the current I.sub.dark is thermally active, namely it varies as a function of the temperature in the form I.sub.dark .alpha.e.sup.-Eg/jkT
where Eg is the width of the gap of the semiconductor, close to the energy of the photon to be detected.
More specifically, the dark current in a photovoltaic detector is given by the following equation:
I.sub.dark =I.sub.diff +E.sub.ZCE
with I.sub.diff =q L.sub.diff .multidot.n.sub.i.sup.2 /.tau..sub.min .multidot.N.sub.dop
and I.sub.ZCE =q n.sub.i d/.tau..sub.min
where:
I.sub.diff is the diffusion current
I.sub.ZCE is the space charge zone current
L.sub.diff is the diffusion length (typically some tens to some hundreds of microns)
.tau..sub.min is the lifetime of the minority carriers
n.sub.i is the intrinsic carrier density
d is the thickness of the space charge zone.
FIG. 1 illustrates a standard photovoltaic detector having a p-n junction in which the lengths L.sub.diff and d are shown.
In these detectors, it is particularly n.sub.1.sup.2 that gives the thermally activated term. Conventionally, by cooling these infrared detectors, the dark current and therefore the associated noise are reduced. Furthermore, since the dark current is proportional to the thickness d of the detector zone, it is judicious to have a very thin detector zone. More specifically, the detectivity limited by the dark current is given by the formula:
D*=.eta..lambda./[u.sup.1/2 hc. (I.sub.dark /qG.sup.2 A).sup.1/2 ]
where
q is the charge of the electron
h is Planck's constant
c is the velocity of light
k is the Boltzmann constant
T is the temperature
G is the gain in photoconduction (G=1 if the component is photovoltaic)
u=2 if the component is photovoltaic
u=4 if the component is photoconductive, and
.eta.=the quantum yield given by the absorption of light in the layer with a thickness d giving: EQU .alpha.=1-e.sup.-.alpha.d
where .alpha. is the coefficient of absorption of the material.
The detectivity D* is therefore proportional to: EQU D*.alpha. .eta.(d)/.sqroot.d
and in the standard case .eta.(d)=1-e.sup.-.alpha.d
FIG. 2 illustrates the development of the detectivity D* as a function of the thickness d. The curve 2a relates to standard quantum detectors in which .eta.(d)=1-e.sup.-.alpha.d, these detectors typically using thicknesses in the range of one micron. The curve 2b pertains to a quantum detector in which .eta.(d) is constant. With this type of detector, it becomes particularly useful to work with a very low thickness d which may be in the range of 0.1 .mu.m and a constant quantum yield .eta., the very low thickness d leading to low dark currents that no longer require any device for the cooling of the detector.